The problem is that the distance from the arbor to the splinter guard is wrong.
This is only an issue when the blade is not perpendicular to the splinter guard.
First make a bevel cut with another guide rail.
If the saw overcuts the splinter guard on the other guide rail then the saw
is out of adjustment and should go back to Festool.
Something is wrong with the mounting blocks (probably) making the distance from the saw to the plate too great. The blade set to 45* should just touch the bottom corner of the rubber strip.
(If the distance was too small the saw blade would pass under the splinter guard when cutting a bevel, and the simple solution would be to add a shim to the mounting block. Some early TS 75 saws had that problem.)
This is something you should get Festool to take care of but if you were on a glacier or some other very remote place you could probably do it yourself.
If the saw cuts as it should on the other guide rail then there is another possibility.
Another way that distance could be thrown off is if the green strips on the guide rail are too tall (or too short in another case, or when worn down). Check that the green strips are properly installed in the groove and not overlapping.
